The aim of this research is to identify long QT syndrome families not linked to one of the previously described loci and to identify mutations in families linked to the LQT1, LQT2 or LQT3 loci. We intend to obtain concurrent electrocardiograms (ECG) and blood samples for leukocyte DNA analysis to examine for linkage between the LQT1, 2, 3 and 4loci on chromosomes 11, 7, 3 and 4 respectively and a long QT interval on the ECG. Identification of families not linked to one of these loci will be important in identifying other genes that can cause this life- threatening disorder. Also, identifying mutations in families linked to one of the previously-described genes will further our understanding of the disease process.